To meet the demand for wireless data traffic that has increased since deployment of 4th Generation (4G) communication systems, efforts have been made to develop an improved 5th Generation (5G) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘Beyond 4G Network’ or a ‘Post Long Term Evolution (LTE) System’. The 5G communication system is considered to be implemented in higher frequency (mmWave) bands (e.g., 60 GHz bands), so as to achieve higher data transmission rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), Full Dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems. In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud Radio Access Networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (COMP), reception-end interference cancellation and the like. In the 5G system, Hybrid Frequency Shift Keying (FSK) and Quadrature Amplitude Modulation (QAM) Modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
The Internet, which is a human centered connectivity network where humans generate and consume information, is now evolving to the Internet of Things (IoT) where distributed entities, such as things, exchange and process information without human intervention. The Internet of Everything (IoE), which is a combination of the IoT technology and the Big Data processing technology through connection with a cloud server, has emerged. As technology elements, such as “sensing technology”, “wired/wireless communication and network infrastructure”, “service interface technology”, and “Security technology” have been demanded for IoT implementation, a sensor network, a Machine-to-Machine (M2M) communication, Machine Type Communication (MTC), and so forth are currently being researched. Such an IoT environment may provide intelligent Internet technology services that create a new value to human life by collecting and analyzing data generated among connected things. IoT may be applied to a variety of fields including a smart home, a smart building, a smart city, a smart car or connected cars, a smart grid, health care, smart appliances and advanced medical services through convergence and combination between existing Information Technology (IT) and various industrial applications.
In line with this, various attempts have been made to apply 5G communication systems to IoT networks. For example, technologies such as a sensor network, Machine Type Communication (MTC), and Machine-to-Machine (M2M) communication may be implemented by beamforming, MIMO, and array antennas. Application of a cloud RAN as the above-described Big Data processing technology may also be considered to be as an example of convergence between the 5G technology and the IoT technology.
Currently, due to the supply of smart phones, data traffic has rapidly increased. In the future, the number of users of smart phones will further increase and the increase in the number of users is expected to further increase data traffic above current levels since application services such as Social Network Services (SNS) and games using the smart phones will be activated more frequently. Particularly, when M2M communication, such as communication between a person and a machine and communication between machines that correspond to a new mobile market beyond communication between people is activated, traffic transmitted to an evolved NodeB (eNB) is expected to increase beyond that which may be handled.
Accordingly, technologies to solve the above problems are needed, and a direct communication between devices is recently spotlighted as the technology. The technology called D2D communication is spotlighted in both a licensed band of mobile communication and an unlicensed band such as Wireless Local Area Network (WLAN).
An LTE-based D2D communication technology may be classified into a D2D discovery and D2D communication. The D2D discovery refers to a series of processes in which one User Equipment (UE) identifies identities or interests of other UEs existing on proximity to the UE or informs other UEs located near the UE of the UE's own identity or interest. At this time, an identity and interest may include an identifier (ID) of the UE, an application ID, or a service ID, and may be variously configured according to a D2D service and operation scenario.
It is assumed that a hierarchical structure of the UE includes a D2D application layer, a D2D management layer, and a D2D transport layer. The D2D application layer may refer to a D2D service application program driven by a UE Operating System (OS), the D2D management layer may perform a function of converting discovery information generated by a D2D application program into a format suitable for a transport layer, and the transport layer may refer to a Physical layer (PHY)/Medium Access Control (MAC) layer of an LTE or Wi-Fi wireless communication standard. At this time, the D2D discovery may be performed by the following process. When a user executes a D2D application program, information for discovery is generated by the application layer and the generated information is transmitted to the D2D management layer. The management layer converts the discovery information received from the application layer into a management layer message. The management layer message is transmitted through the transport layer of the UE, and UEs receiving the management layer message perform a reception operation in a reverse order of the transmission process.
The D2D communication corresponds to a communication method in which UEs directly transmit traffic therebetween without passing through infrastructure such as an eNB or Access Point (AP). At this time, the D2D communication may be performed based on a D2D discovery process after the D2D discovery process is performed, or may be performed without the D2D discovery process. The D2D discovery process before the D2D communication may or may not be necessary according to a D2D service and operation scenario.
The D2D service scenario may be largely classified into a commercial service (or a non-public safety service) and a public safety service. Each of the services may include innumerable use examples, but representatively include, for example, an advertisement, an SNS, a game, and a public safety service.
1) Advertisement: a communication network operator that supports D2D may allow pre-registered stores, cafes, theaters, restaurants, and the like to advertise identities thereof to D2D users located within a short distance by using D2D discovery or D2D communication. At this time, the interest may be a promotion of advertisers, event information, or discount coupons. When the corresponding identity matches the interest, the user may visit a corresponding store and acquire much more information by using an existing cellular communication network or D2D communication. In another example, an individual user may discover a taxi located nearby the UE through a D2D discovery and exchange data on a destination or fare information of the UE through existing cellular communication or D2D communication.
2) SNS: the user may transmit an application of the user and interest in the corresponding application to other users located in a nearby area. At this time, the identity or interest used for the D2D discovery may be a friend list of the application or an application ID. The user may share contents such as pictures or videos possessed by the user with nearby users through D2D communication after the D2D discovery.
3) Game: the user may discover users and a game application through the D2D discovery process in order to play a mobile game with users located nearby and perform D2D communication to transmit data required for the game.
4) Public safety service: policemen or firefighters may use the D2D communication technology for the purpose of public safety. That is, when an existing cellular network is partially damaged due to an emergency situation such as a fire or a landslide or a natural disaster such as an earthquake, an eruption or a volcano, or a tsunami and thus cellular communication is not possible, policemen or firefighters may find adjacent companions through the D2D communication technology or share their own emergency information with adjacent users.
A 3rd Generation Partnership Project (3GPP) LTE D2D standardization which is currently discussed is progressed for both the D2D discovery and the D2D communication, but there is a difference in standardization ranges. The D2D discovery is performed for the commercial purpose, and should be designed to operate only in network coverage. That is, the D2D discovery is not supported in a condition where the eNB does not exist (or beyond the coverage of the eNB). The D2D communication is performed for the purpose of a public safety service rather than the commercial purpose, and should be supported in all of conditions such as in-network coverage, out-of-network coverage, and partial network coverage (communication under a condition where some UEs exist in the network coverage and some UEs exist out of the network coverage). Accordingly, in the public safety service, the D2D communication should be performed without the support of the D2D discovery.
In the currently discussed LTE D2D standardization, both the D2D discovery and the D2D communication are performed in an LTE uplink subframe. That is, a D2D transmitter transmits a D2D discovery signal and data for D2D communication in the uplink subframe and a D2D receiver receives the D2D discovery signal and the data in the uplink subframe. Currently in the LTE system, since the UE receives data and control information from the eNB through downlink and transmits data and control information to the eNB through uplink, operations of the D2D transmitter/receiver may be different from those in LTE. For example, the UE which does not support a D2D function has an Orthogonal Frequency Division Multiplexing (OFDM)-based receiver to receive downlink data and control information from the eNB, and requires a Single Carrier-FDM (SC-FDM)-based transmitter to transmit uplink data and control information to the eNB. However, since the D2D UE is required to support both a cellular mode and a D2D mode, the D2D UE should also have a separate SC-FDM receiver to receive D2D data and control information through uplink as well as the OFDM-based receiver for receiving downlink data from the eNB and the SC-FDM-based transmitter for transmitting data or control information, or D2D data and control information to the eNB through downlink. Currently, two types of D2D discovery methods are defined according to a resource allocation method.
1) Type 1 discovery: the eNB broadcasts an uplink resource pool which may be used for the D2D discovery to all D2D UEs within a cell managed by the eNB via a System Information Block (SIB). At this time, the size of resources which may be used for D2D (for example, x consecutive subframes) and a period of resources (for example, repetition every y seconds) may be informed. D2D transmission UEs having received the uplink resource pool distributively select resources used by themselves and transmit D2D discovery signals. There may be a variety of methods to distributively select the resources by the D2D transmission UEs. For example, a simplest method may be random resource selection. That is, the D2D transmission UE which desires to transmit a D2D discovery signal randomly selects resources to be used directly by the D2D transmission UE within a type 1 discovery resource area acquired through the SIB. Another resource selection method may be a method of selecting resources by the UE based on energy sensing. That is, the D2D transmission UE which desires to transmit a D2D discovery signal senses every level of all Resource Blocks (RBs) existing within the type 1 discovery resource area acquired through the SIB during a predetermined interval, selects an RB having a lowest energy level or an RB having an energy level equal to or smaller than a particular threshold, or sort RBs having energy levels equal to or smaller than a particular threshold and then randomly select resources among the sorted RBs. The D2D transmission UE having selected resources transmits the discovery signal to the selected RBs in the next type 1 discovery resource area after the energy sensing interval. The D2D reception UEs should receive (decode) all D2D discovery signals in the resource pool included in SIB information. For example, the D2D reception UEs, which recognize that x consecutive subframes repeat every y seconds through the SIB decoding, perform the decoding on all RBs allocated for the D2D discovery within the x consecutive subframes. In the type 1 discovery, all D2D UEs in a cellular Radio Resource Control (RRC)-Idle mode and an RRC_Connected mode may transmit/receive discovery signals.
2) Type 2 discovery: the eNB informs of a discovery signal resource pool which the D2D reception UEs should receive through the SIB. Discovery signal transmission resources for the D2D transmission UEs are scheduled by the eNB (that is, the eNB instructs the D2D transmission UEs to perform transmission in a particular time-frequency resource). At this time, the scheduling of the eNB may be performed through a semi-persistent scheme or a dynamic scheme, and the D2D transmission UE should make a request for allocating D2D transmission resources such as a Scheduling Request (SR) or a Buffer Status Report (BSR) for such an operation to the eNB. Accordingly, in the type 2 discovery, all D2D UEs should be in the RRC-Connected mode. That is, the D2D transmission UEs in the RRC-Idle mode should switch to the RRC_Connected mode through a random access process in order to make the request for allocating D2D transmission resources. Allocation information of the D2D transmission resources of the eNB may be transmitted to each of the D2D transmission UEs through RRC signaling or through an (enhanced) Physical Downlink Control CHannel ((e)PDSCH).
A D2D communication method may be classified into two types according to resource allocation like the D2D discovery method.
1) Mode 1: the eNB or a Release 10 relay directly informs a D2D transmitter of resources to be used by the D2D transmitter to transmit data and control information for D2D communication. Further, by using the SIB, the eNB informs of a D2D signal resource pool which the D2D reception UE should receive.
2) Mode 2: based on resource pool information for transmission of the data and control information acquired by the D2D transmitter through the SIB or a separate control channel (Physical D2D Synchronization CHannel: PD2DSCH), the D2D transmitter solely distributively selects and transmits resources within the corresponding resource pool. At this time, a method of selecting resources by the D2D transmitter may include the random resource selection method or the energy-sensing based resource selection method as described in the type 1 discovery.
In a cellular system such as LTE, various interferences may occur when D2D communication is supported. Such various interferences may be caused by characteristics of the cellular system described below.
Transmit Power Control (TPC) in Cellular System
In a cellular system, in uplink transmission of the UE, the eNB reduces interference caused to another cell, increases a battery life of a cellular UE, and controls UE transmission power to receive data and control information from each UE with proper power. In order to control uplink transmission power of UEs, the eNB may inform the UEs of various parameters required for controlling the transmission power or the UEs may predict some parameters by themselves to determine their own transmission power and configure transmission power. In order to determine the parameters, with assistance from the UE, the eNB may measure channel quality (received signal strength) between the eNB and the UE and channel quality (for example, interference signal strength) which may influence the eNB and the corresponding UE and reflect the measured channel quality to control the transmission power. For example, in the LTE system, transmission power PPUSCH(i) of a Physical Uplink Shared CHannel (PUSCH) corresponding to a physical channel for uplink data transmission in an ith subframe of the UE is defined as Equation 1 below.
                                          P            PUSCH                    ⁡                      (            i            )                          =                  min          ⁢                                    {                                                                                                                  P                        CMAX                                            ⁡                                              (                        i                        )                                                                                                                                                                                                          10                          ⁢                                                                                    log                              10                                                        ⁡                                                          (                                                                                                M                                  PUSCH                                                                ⁡                                                                  (                                  i                                  )                                                                                            )                                                                                                      +                                                                              P                                                          0                              ⁢                                                              _                                ⁢                                PUSCH                                                                                                              ⁡                                                      (                            j                            )                                                                          +                                                                              α                            ⁡                                                          (                              j                              )                                                                                ·                          PL                                                +                                                                              Δ                            TF                                                    ⁡                                                      (                            i                            )                                                                                              =                                              f                        ⁡                                                  (                          i                          )                                                                                                                                }                        ⁡                          [              dBM              ]                                                          Equation        ⁢                                  ⁢        1            
Parameters for controlling power are defined below.
PCMAX(i): Maximum transmission power of the UE in an ith subframe
MPUSCH(i): The number of RBs allocated by the eNB for PUSCH transmission in the ith subframe.
P0_PUSCH(j): Parameter including PO_NOMINAL_PUSCH(j)+PO_UE_PUSCH(j), (where j=0: semi-persistent grant, j=1: dynamic scheduling grant, j=2: random access response grant), and corresponds to a value of which the eNB informs the UE through higher layer signaling. When j=0 or j=1, PO_NOMINAL_PUSCH(j) is a cell-specific value of 8-bit information and has a range of [−126, 24] dB. Further, PO_UE_PUSCH(j) is a UE-specific value of 4-bit information and has a range of [−8, 7] dB. When j=2, PO_UE_PUSCH(j) is 0. The cell-specific value is transmitted by the eNB through the SIB and the UE-specific value is transmitted to the UE by the eNB through dedicated RRC signaling.
α(j): Value for compensating for a path-loss and a cell-specific value corresponding to one of {0, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1} informed by the eNB through 3-bit information when j=0 or j=1. When j=2, α(j)=1 is used.
PL: downlink path-loss measured by the UE.
ΔTF(i): Compensation value according to a Modulation Coding Scheme (MCS) to be used in the ith subframe
f(i): accumulated power control or absolute power control function (whether to use the accumulated power control or the absolute power control function is determined through higher layer signaling). For example, according to whether accumulation-enabled is on or off, the accumulated power control or the absolute power control function is used.
Accumulated power control: f(i)=f(i−1)+δPUSCH(i−KPUSCH)
Absolute power control: f(i)=δPUSCH(i−KPUSCH)
The eNB informs the UE of δPUSCH through a Transmit Power control (TCP) command within Downlink Control Information (DCI) transmitted through a downlink control channel (Physical Downlink Control CHannel (PDCCH). At this time, δPUSCH is transmitted by DCI of the downlink PDCCH before a KPUSCH subframe and reflected in power control for uplink subframe transmission. δPUSCH may have values of −1, 0, 1, and 3 [dB] in DCI format 0, DCI format 3, and DCI format 4, and may be expressed in 2 bits. In DCI format 3A, δPUSCH may have values of −1 and 1 [dB], and may be expressed in 1 bit. KPUSCH=4 in a Frequency Division Duplexing (FDD) system, and KPUSCH has various values according to a TDD DL/UL configuration in Time Division Duplexing (TDD) system.
As known from the above equation, the UE receives P0-NOMINAL_PUSCH(j) and α(j) which are cell-specific parameters, P0_UE_PUSCH(j) and ΔTF(i) which are UE-specific parameters, and j from the eNB through higher layer signaling in the LTE system. Further, the UE may acquire values required for f(i) through a PDCCH corresponding to a downlink control channel.
Timing Advance (TA)
In the cellular communication according to the related art, the eNB performs TA to receive data and control information, which are transmitted from UEs located at different positions within the cell managed by the eNB through the uplink, at the same period of time. At this time, a TA value, which the eNB transmits to the UE, may vary depending on a Round Trip Delay (RTD) between the eNB and the UE. For example, since UEs located nearby the eNB have a small RTD, the eNB informs the corresponding UEs of a small TA value. On the contrary to this, since UEs located far away from the eNB have a large RTD, the eNB informs the corresponding UEs of a large TA value.
The UEs having received the TA values drive timers installed in the UEs, and follow a TA command received from the eNB before the timers thereof expire unless there is a separate command from the eNB. That is, before the timer expires, data and control information, which the UE transmits to the eNB through the uplink, are based on the corresponding TA value.
Cyclic Prefix (CP) length
The LTE system supports two types of CP lengths (normal CP and extended CP). The CP lengths may be configured by operators according to cell coverage and cell channel environment. For example, when the cell coverage is small and channel delay spread is narrow, the normal CP may be used. In contrast, when the cell coverage is large and channel delay spread is wide, the extended CP may be used. In the LTE system, the downlink CP length is provided to the UE without special signaling, and each UE may blindly detect the downlink CP length during a Primary Synchronization Signal (PSS)/Secondary Synchronization Signal (SSS) detection process for downlink synchronization with the eNB. The uplink CP length is configured to all UEs within the cell through SIB2. That is, the LTE system assigns freedom to the system design in order to differently operate the uplink CP length and the downlink CP length.
In the cellular (LTE) system according to the related art, the UE receives data and control information from the eNB through the downlink and transmits data and control information to the eNB through the uplink. However, in an LTE-based D2D system, a D2D discovery signal and D2D communication are performed in an uplink subframe. That is, a D2D transmission UE transmits a D2D discovery signal and data/control information for D2D communication in an uplink subframe, and a D2D reception UE receives a D2D discovery signal and data/control information for D2D communication in an uplink subframe. With respect to resources for the D2D discovery signal and the D2D communication, a PUSCH for uplink data transmission of the cellular UE of the related art or an uplink feedback channel of the UE may be frequency-division multiplexed and used in the same subframe as that of a Physical Uplink Control CHannel (PUCCH).
When D2D resources and resources of the cellular UE of the related art are frequency-division multiplexed and used in the same subframe, if the D2D transmission UE uses maximum transmission power in order to increase the coverage (or range) of the D2D discovery signal and D2D communication, transmission signals (discovery signal and communication signal) of the D2D UE causes an in-band emission problem in the eNB, which receives the PUCCH or PUSCH transmitted from the cellular UE of the related art. That is, the eNB performs a power control to allow an eNB receiver to consistently receive the PUCCH (or PUSCH) transmitted by the cellular UE through the uplink without escaping from a dynamic range of an Automatic Gain Control (AGC) gain. At this time, when a power strength of a discovery signal or a D2D communication signal transmitted by the D2D transmission UE located nearby the eNB is great, the AGC gain of the eNB receiver is controlled and the cellular UE performs the power control. Accordingly, the PUCCH (or PUSCH) transmitted to the eNB through the uplink may escape from the dynamic range of the AGC of the eNB receiver and thus may not be received. Such a phenomenon may be called In-Band Emission (IBE).
Based on the Release-12 D2D standard, the PUSCH transmitting the D2D signal, which is transmitted by the D2D UE, and the PUCCH, which is transmitted by the cellular UE of the related art, may be frequency-division multiplexed and used in the same subframe. The PUCCH of the cellular UE of the related art is transmitted based on TA according to a command of the eNB. For example, the cellular UE located nearby the eNB may perform transmission with a small TA value, and the cellular UE far away from the eNB may perform transmission with a large TA value. However, in D2D type 1 discovery or D2D mode 2 communication, the D2D signal is transmitted based on a downlink transmission reference time, not based on an uplink transmission reference time (based on TA) to support the RRC_Idle mode UE. That is, in the RRC_Idle mode, uplink synchronization is not maintained, and only random access performed to move to RRC_CONNECTED is possible in uplink transmission. Accordingly, the D2D signal is transmitted based on the downlink time after the downlink PSS/SSS, which is not transmitted through the uplink, is received from the eNB and downlink synchronization is performed. In this case, since the PUCCH is transmitted according to the uplink reference time based on TA, and the D2D PUSCH is transmitted according to the downlink reference time, if the PUCCH and the D2D PUSCH are frequency-division multiplexed and used in the same subframe, the D2D PUSCH may cause an Inter-Carrier Interference (ICI) problem in reception of the PUCCH of the eNB.
If the D2D PUSCH and the cellular PUSCH of the related art are time-division multiplexed and used, the D2D PUSCH may give an Inter-Symbol Interference (ISI) problem to the cellular PUSCH. For example, when it is assumed that the D2D PUSCH is transmitted in an nth subframe according to the downlink reference time and the cellular PUSCH is transmitted in an n+1th subframe according to the uplink reference time, if the DD PUSCH receives the PSS/SSS with a T1 propagation delay, the D2D PUSCH in the nth subframe is received by the eNB while having a 2*T1 propagation delay since the D2D PUSCH is transmitted according to the downlink reference time. When the propagation delay is longer than a CP length of the n+1th subframe, the D2D PUSCH may cause interference in the cellular PUSCH and thus the eNB may not smoothly receive the cellular PUSCH.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.